How can forced induction increase engine efficiency?

(I have looked in some detail at how superchargers and turbochargers work. I also searched the forum but haven't found anything that helps me make sense of my confusion.)

I have heard that turbochargers increase engine efficiency (not just power) and fuel economy. How can this be (if it is true)? Forced induction adds more air to an engine, or the availability of air to be sucked in the combustion chamber. The amount of fuel must be increased to compensate for the extra air. Because more air and fuel is required doesn't this keep the engine efficiency just where it would be without a turbocharger since more input (A/F mix) is required to achieve more output (Power) with a turbocharger? Additionally, turbochargers must add some backpressure right?

Is there something I am not thinking of? Does it come down to little technicalities that I'm not seeing? Am I not seeing the big picture?

Thanks.

MagicRat

06-28-2010, 08:43 PM

Engine-driven superchargers do not increase efficiency. When they are building boost, they will increase cylinder pressure and thus increase thermodynamic efficiency (much they way an increase in compression ratio gives you better fuel economy and power)
But the parasitic losses related to spinning the compressor more than offsets such increases.

Turbochargers will increase efficiency (and power) but only when the intake manifold is positively pressurized.
Turbos spin because they are driven by the otherwise-wasted thermal expansion energy found in exhaust gases. They can return some of the energy by pressurizing the intake manifold. When the intake valve opens, the pressurized air in the intake manifold flows into the cylinder and pushes the piston down. In a normal engine, that piston expends energy sucking air through the valve and building engine vacuum.
Also, turbos increase cylinder pressures and produce thermodynamic efficiency.

Therefore, turbos are efficient in gasoline engines when you are using wide throttle openings and developing lots of power, but lose efficiency as the throttle is closed... which is not good. At light throttle openings, you want more effciency, such as highway cruising, just when a gas turbo is not helping efficiency.
Therefore, you will see many more small gasoline turbo engines in the future, which are designed to develop boost during normal operation to boost fuel efficincy.

Turbos improve efficiency best in diesel engines. Since diesels have no throttle blades, the spinning turbo can boost a diesels efficiency most of the time, even in low-power operation.

534BC

06-29-2010, 04:53 PM

Think of an engine as a compressor and in it's best day being 100% VE
Realistically it may be from 70-90% because we aren't filling the cylinders 100% all the time.

Think of then adding air pressure to the inlet of the engine, and now it is able to fill the cylinders over 100%

At 2 bar it could be said that we now have "twice as big of an engine" as is actual size and theoretically the VE could approach 200% now.

ps VE only works for WOT.

curtis73

07-07-2010, 01:07 AM

There are more factors than just the positive pressure.

The positive pressure does increase cylinder peak pressure which superficially increases flame front speed and (depending on fuel tune) will cause a more complete combustion. That simply means that - of the fuel that is ingested, more is converted into heat energy than in an engine with lower cylinder pressures.

Another common design factor with forced induction is reduced cam overlap. This reduces intake reversion/dilution which allows more accurate fuel metering, and almost always increased MPG.

The other thing to consider is this: A 2L engine with 15 psi of boost can theoretically mimic the output of a 4L enging without boost. While both engines might be well-matched at the races, on the street a car is driven almost always between 0-70% throttle. At the race, a car is almost always at full throttle. On the street that means the 2L turbo engine is operating most of its time as a non-boosted 2L engine. The 4L engine is always a 4L engine.

A supercharger is driven as a function of engine RPM. It provides peak boost regardless of your accelerator position. A turbo only provides boost as a function of how much right foot you put into it. Therefore a turbo 2L engine can be a fuel-sipping 2L engine if you keep your foot out of it, or it can (on demand) act like a fuel-guzzling 4L engine at full throttle/boost.

The efficiency term is misleading. There are three main "efficiencies" involved with engine tuning:
1) Volumetric efficiency - a comparison of atmospheric density to the mass of air actually ingested in the cylinder
2) Thermal efficiency - a comparison of the BTUs stored in the fuel compared to the actual amount of BTUs released as power at the crankshaft
3) Fuel efficiency - a combination of 1 and 2 along with about 6 trillion other factors that can't be quantified

Forced induction can increase #1 (VE) theoretically by how much pressure it introduces less the amount of inefficiency it invokes by heating the air it pressurizes. It can also increase #2 (thermal) since it increases peak cylinder pressures and the amount of BTUs released by the fuel.

Whether or not it increases #3 is up to those 6 trillion other factors. All things being equal, (at wide open throttle) a 4L naturally aspirated engine might consume the same amount of fuel as a 2L turbo at 15 psi, but how they actually consume fuel is mostly up to how the driver uses his or her right foot.

KiwiBacon

07-17-2010, 01:41 AM

The big saver in efficiency with turbocharged petrol engines is downsizing. This is what Curtis was getting at with the 2L turbo engine compared to a 4L engine.

The 4L engine always has the pumping and friction losses of a 4L engine, where the 2L turbo engine has a little higher losses than a 2L non turbo engine, but has big savings on the 4L.
So it saves fuel through operating at higher load point, reduced pumping losses and by recovering some otherwise wasted exhaust energy.

Now if you want to talk real engines (diesel) pretty much all the turbocharging downsides (reduced compression ratio, no boost at cruise etc etc) are gone. Completely gone.
A turbocharged diesel is more efficient in all modes than a non turbo diesel. The only downsides are the cost and complexity in adding the turbo (oil lines, extra intake and exhaust plumbing).